Silicon clathrate Si46 refers to crystalline Si with a regular arrangement of 20-atom and 24-atom cages fused together through 5 atom pentagonal rings (Type I clathrate). It has a simple cubic structure with a lattice parameter of 10.335 Å and 46 Si atoms per unit cell. FIG. 1 illustrates the cage structure of the Si46, which belongs to the Space Group Pm3n and Space Group Number 223. The crystal structure of the silicon clathrate (Si46) is different from the common form of crystalline Si (c-Si), which is diamond cubic with a lattice parameter of about 5.456 Å and belongs to the Space Group Fd3m, Number 227.
Another form of silicon clathrate is Si34 (Type II clathrate) that consists of crystalline Si with a regular arrangement of 20 atoms and 28 atom cages fused together through five-atom pentagonal rings. Type II Si34 clathrate has a face-centered cubic (fcc) structure, with 34 Si atoms per fcc unit cell. The Si34 clathrate has a lattice parameter of 14.62 Å and belongs to the Space Group Fd3m, Number 227. Type II silicon clathrate is sometimes referred to as Si136 since the compound contains four fcc unit cells. A third form of silicon clathrate is a modification of the Si46 type formed by removing four atoms from the 24-atom cages.